The production of liquid crystal displays such as, for example, active matrix liquid crystal display devices (AMLCDs) is very complex, and the properties of the substrate glass are extremely important. First and foremost, the glass substrates used in the production of AMLCD devices need to have their physical dimensions tightly controlled. The downdraw sheet drawing processes and, in particular, the fusion process described in U.S. Pat. Nos. 3,338,696 and 3,682,609, both to Dockerty, are capable of producing glass sheets that can be used as substrates without requiring costly post-forming finishing operations such as lapping and polishing. Unfortunately, the fusion process places rather severe restrictions on the glass properties, which require relatively high liquidus viscosities.
In the liquid crystal display field, thin film transistors (TFTs) based on poly-crystalline silicon are preferred because of their ability to transport electrons more effectively. Poly-crystalline based silicon transistors (p-Si) are characterized as having a higher mobility than those based on amorphous-silicon based transistors (a-Si). This allows the manufacture of smaller and faster transistors. P-Si displays are at the core of state-of-the-art handheld devices. The p-Si thin film transistor array consumes very low power, permits very fine features (critical for small displays), and provides high brightness.
The process used to make p-Si TFTs invariably includes a thermal excursion to quite high temperature to encourage the silicon to crystallize. In some processes, temperature alone is used to produce crystallization, and in such processes the peak temperatures are very high, very typically greater than 650° C. compared to the 350° C. peak temperatures employed in the manufacture of a-Si transistors. At these temperatures, most AMLCD glass substrates undergo a process known as compaction and will deform excessively unless supported from below. Compaction, also referred to as thermal stability or dimensional change, is an irreversible dimensional change (shrinkage or expansion) in the glass substrate due to changes in the glass' fictive temperature. The magnitude of compaction depends both on the process by which a glass is made and the viscoelastic properties of the glass. In the float process for producing sheet products from glass, the glass sheet is cooled relatively slowly from the melt and, thus, “freezes in” a comparatively low temperature structure into the glass. The fusion process, by contrast, results in very rapid quenching of the glass sheet from the melt, and freezes in a comparatively high temperature structure. As a result, a glass produced by the float process possesses less compaction when compared to glass produced by the fusion process. In the glass product itself, the compaction ultimately may produce poor registry with the color filter and, if large enough, adversely affect device performance. Thus, it would be desirable to minimize the level of compaction in a glass substrate that is produced by a downdraw process. A commercial glass product, Jade® (Corning Incorporated, Corning N.Y.), was developed expressly to address this problem. It has a very high annealing point compared to conventional amorphous silicon substrate glasses, and thus shows low compaction even when reheated above the strain point of conventional amorphous silicon substrates.
While the Jade® product has proven sufficient for many p-Si processes, there is still a demand for even lower levels of compaction and/or the capability of withstanding heat treatments at even higher temperatures, sometimes in excess of 700° C. Annealed low annealing point p-Si substrate glasses can be optimized to provide these lower levels of compaction but their deformation at elevated temperature is only marginally improved with this increased anneal. The Jade® product has better deformation at elevated temperatures than annealed low annealing point glasses but has too much compaction for some applications and may need even more resistance to deformation under the extreme conditions of some newly developed cycles. In order to provide the desired lower compaction in existing processes as well as enabling the development of new higher temperature processes, a glass with an annealing point in excess of 785° C. is desired.